(a) Technical Field
The present invention relates to a method for preparing ultrahigh-purity silicon carbide (SiC) powders, more particularly to a method for preparing ultrahigh-purity silicon carbide granular powders by preparing a gel wherein a silicon compound and a carbon compound are uniformly dispersed via a sol-gel process using a liquid state silicon compound and a solid or liquid state carbon compound, which have varying purities, as raw materials, preparing a silicon dioxide-carbon (SiO2—C) composite by pyrolyzing the prepared gel, preparing silicon carbide-silicon dioxide-carbon (SiC—SiO2—C) composite powders via two-step carbothermal reduction processes of the prepared silicon dioxide-carbon composite, adding a silicon metal and then conducting both the direct reaction between the silicon metal and carbon and carbothermal reduction at the same time by heat treating, thereby growing the synthesized silicon carbide particles with a high yield.
(b) Background Art
Silicon carbide (SiC) is a non-oxide-based ceramic material. Since it exhibits superior properties including oxidation resistance, corrosion resistance, wear resistance, thermal shock resistance, high-temperature strength, etc. due to the strong covalent bond between silicon and carbon, it is widely used as a structural material for high-temperature applications. In general, SiC has been mainly used as abrasives, heating elements, refractory materials, etc. Furthermore, the importance of SiC as a core material for high-temperature industries has been widely accepted because of-its superior high-temperature stability and chemical resistance.
Recently, materials with large band gap and high dielectric breakdown property are required in the power semiconductor market to reduce the size of a device and to minimize the power loss. In this regard, SiC is recognized as the optimal material for high-output power devices and power devices for high temperature applications.
With its excellent thermal properties as well as superior semiconductor/electrical properties, SiC single crystals can replace Si single crystal for high power semiconductors of which the maximum operation temperature is 250° C. SiC single crystals are suitable for semiconductor devices operating under harsh environments because they are chemically stable, strongly resistant to radiation, and so forth. Also in the LED industry, SiC single crystals are used as an LED substrate for growing GaN. Accordingly, demand on ultrahigh-purity SiC single crystals is increasing with the expanding LED market.
Methods for growing SiC single crystals include a liquid phase epitaxy (LPE) method of growing SiC single crystals from a Si melt, a CVD method and a physical vapor transport (PVT) method. At present, 6-inch SiC wafers manufactured by the PVT method are commercially available. For the growth of SiC single crystals by the PVT method, high-purity SiC granular powders are used. The ultrahigh-purity SiC powders are supplied exclusively by specific companies involved in SiC single crystal manufacturers and there is no open market established for the ultrahigh-purity SiC powders.
Korean Patent Publication No. 10-2011-0021530 (patent document 1) discloses a technology of mixing a solid silica and a solid carbon source and synthesizing high-purity SiC powders by conducting the carbothermal reduction at 1600-1900° C. to manufacture high-purity SiC granular powders used for preparation of SiC single crystals by the PVT method. The International Patent Publication No. WO 2014-061898 (patent document 2) discloses a technology of synthesizing high-purity α-phase SiC granular powders having an average particle size of 75-110 μm and containing less than 10 ppm of impurities by heat-treating high-purity SiC powders prepared as described above at 2000-2200° C. under an inert atmosphere. And, US Patent Publication No. 2009-0220788 (patent document 3) discloses a technology of synthesizing ultrahigh-purity α-phase SiC granular powders containing extremely low contents of nitrogen, boron and aluminum by heat-treating a silicon metal and a high-purity solid carbon at 1200° C. under a vacuum atmosphere for 12 hours and then raising temperature to 2250° C. at a pressure of 10−5 torr or lower. However, the methods presented in the patent documents 2 and 3 are disadvantageous in that the cost of powder synthesis is increased because the heat treatment is conducted at high temperatures of 2000-2250° C. and that a synthesis yield is low because the granular powders are obtained through β to α phase transition.
U.S. Pat. No. 4,702,900 (patent document 4) discloses a technology of synthesizing β-phase ultrahigh-purity SiC granular powders by preparing a silicon dioxide-carbon precursor from a silicon alkoxide and a carbon compound and heat-treating under a vacuum condition or an inert gas condition such as argon (Ar). And, U.S. Pat. No. 5,863,325 (patent document 5) discloses a heat treatment process optimized for using high-purity β-phase SiC granular powders containing metal impurities of several ppm prepared as described above as a source for single crystals. In the patent document 5, after the β-phase SiC powders are prepared at 1800° C., the powders are treated through 3-6 thermal cycles at 1900-2100° C. to grow the particle size to around 200 μm. However, because of long heat treatment time, the production cost of the SiC powders is high.
U.S. Pat. No. 6,627,169 (patent document 6) discloses a technology of controlling the average particle size of β-phase SiC powders by measuring the amount of CO gas generated during carbothermal reduction. Although SiC powders synthesized in the patent document 6 are of-ultrahigh purity containing metal impurities at a concentration of 0.01 ppm or lower, synthesis yield of SiC powders is low compared with that of starting source materials.
In addition, Korean Patent Publication No. 10-2013-0122476 (patent document 7) discloses a method of manufacturing SiC powders by heat-treating a silicon source such as SiO2, Si, silica sol, etc. and a carbon source such as a phenol resin, sugar, a polymer, etc. at various temperature ranges. And, Korean Patent Publication No. 10-2013-0072067 (patent document 8) discloses a method of manufacturing SiC powders by preparing SiC powder compacts by compressing fine SiC powders, preparing SiC powders aggregate by crushing the same, and then preparing SiC granular powders with a size control by heat-treating the same.
However, it is difficult to economically prepare SiC powders with high purity using the above-described existing methods.